ABSTRACT Systemic lupus erythematosus (SLE) is an autoimmune disease of unknown etiology. The pathogenesis is partly attributed to compartmentalized oxidative stress inside and outside the immune system. The proposed studies will focus on a critical gap in knowledge - how metabolic pathways that neutralize oxidative stress control autoimmunity in SLE. The central hypothesis for this project is based on comprehensive metabolome studies that have showed a dominant impact of SLE on the pentose phosphate pathway (PPP) in lymphocytes of patients and T cells of lupus-prone mice undergoing lineage polarization; the results of which mimic the deficiency of transaldolase (TAL), a rate-limiting enzyme of the PPP. Lupus-prone mice exhibit activation of the mechanistic target of rapamycin (mTOR) and mitochondrial oxidative stress in the liver and antiphospholipid antibody (aPL) production prior to the onset of nephritis. Similar to lupus-prone strains, mice lacking TAL exhibit mTOR activation and overexpression of NDUFS3, a subunit of complex I in the mitochondrial electron transport chain (ETC) that triggers the production of reactive oxygen intermediates (ROI) and aPL, both of which respond to rapamycin treatment. TAL deficiency blocks the glycosylation and secretion of PON1 by the liver. This is attributed to carbon trapping in the PPP and depletion of UDP-GlcNAc which are also detectable in SLE patients and mice. Although PON1 loss in the plasma has been connected to aPL production and demonstrated in SLE, antiphospholipid syndrome (APS), and liver diseases, the underlying mechanisms remain unknown. Therefore, the Specific Aims will test our working hypothesis that TAL inactivation i) elicits cell type-specific carbon sequestration in the PPP and limits substrates for NADPH and GSH production and metabolism through the ETC and thus triggers a compensatory accumulation of oxidative stress-generating mitochondria, mTOR pathway activation and pro-inflammatory lineage skewing in the immune system; and ii) limits the availability of UDP-GlcNAc for glycosylation and secretion of PON1 by the liver, which in turn trigger aPL production in SLE and TAL deficiency. Under Aim 1, we will test the hypothesis that TAL-regulated carbon flux through the PPP causes cell-type specific accumulation of sedoheptulose 7-phosphate, depletion of NADPH and GSH, and redox-mediated mTOR activation to promote the expansion Th17, Tfh, and DN T cells and constriction of CD8 EMT cells and Tregs in SLE patients. Under Aim 2, we will delineate T-cell intrinsic metabolic checkpoints that control systemic autoimmunity in lupus-prone mice. Under Aim 3, we will determine the role of hepatocyte- derived oxidative stress in aPL production, pro-inflammatory lineage skewing in the immune system and lupus pathogenesis. The proposed research is significant because it will establish new, compartmentally defined metabolic checkpoints of autoimmunity with broad translational relevance for the pathogenesis and treatment of SLE. The approach is innovative as it will employ genetic checkpoints of oxidative stress and high-resolution stable isotope tracing of metabolic pathways to delineate lupus pathogenesis.